Wireless transmission of power typically performed with a magnetic device such as a transformer has been known in the industry for many decades and first demonstrated by Nicola Tesla over one hundred years ago. Tesla used a very high voltage across a coil or winding to light a lamp several feet away. Wireless power systems disclosed many decades ago suffered from many limitations, foremost of which was very poor coupling between the transmitting and receiving coil of the transformer. In recent years, wireless power systems have been developed that use resonant operation to boost the coupling between transmitting and receiving coils.
The standard modern wireless power system uses two planar coils (also referred to as “windings”), one coil for the power transmitter and one coil for the power receiver. Power transfer is maximized by adjusting the transmitting frequency to system resonance or by adjusting system resonance to a fixed transmitting frequency. In many applications, transmitting frequency must be fixed or remain within tight limits for various reasons, so the system resonance must then be adjusted by changing system resonance.
The standard method of adjusting system resonance is to use semiconductor switches to place capacitors into or out of the resonant tank. The resulting switched capacitor architecture is expensive both due to the expensive semiconductor switches required to switch the capacitors into and out of the circuit, and the capacitors themselves, if placed onto the substrate of an integrated circuit can also add cost to the integrated circuit. Furthermore, the switched capacitor architecture allows for only a few limited number of steps in capacitance value, thus making it impossible to tune exactly to resonance.